I can assure you that with proper form it is safe, provided you take the proper precautions. I just squatted 200kg without any spotters a couple weeks ago. Deadlifts are less of a challenge, because it is easier to drop the lift.
- Use safeties with your squat rack (like these)
- Practice dumping the bar on the safeties with a weight you are confident you can handle
- Think "superman" and stick your chest out--this helps keep your back tight and in a stronger position
When working out alone, safeties are an absolute necessity. You can use them on both squats and bench press. This makes sure you can escape out from under the bar without it pinning you down.
As to building confidence with heavier weights, I've found a couple things help:
- Squat walkouts: load up a couple 20kg plates more than what your top squat is, unrack the bar, stand for a few seconds, and re-rack the bar.
- Lockouts: practice the top part of a movement with a heavier weight, but one you can handle. It works with bench and deadlifts. It's a partial movement, so set your safeties at the lowest point you want, and then lock the bar out.
The squat walkouts do a couple things for you. They let you get used to a heavy bar on your back, which helps strengthen your core. You aren't trying to do a negative (just the lowering motion), but it lets you know you can put much heavier weights on your back without injury. The lockouts function similarly, but they also help you finish the lift confidently.
What is most important, and I can't stress this enough, is to make sure you aren't rounding the lumbar area. On deadlifts, a bit of rounding in the thoracic area (upper back) is OK, and a good number of the strongest power lifters (100% RAW, IPF, and other federations) have some rounding there. The lumbar region (lower back) is where the biggest risk of injury is. As long as you concentrate on locking the lower lumbar, and hinge at the hips, you will avoid any injury. Lastly, just don't get ahead of yourself. I didn't get to that 200kg squat overnight, and I made the mistake a few times of trying to increase faster than I was able. While I avoided injury, I did push myself into a state of fatigue more than once that required me to slow down my rate of increase.